A hyperiid amphipod acts as a trophic link between a scyphozoan medusa and juvenile Chinook Salmon

Abstract

Gelatinous zooplankton (GZ) can impact productivity of marine ecosystems through competition with small pelagic fish and predation on early life history stages of species from multiple trophic levels. Nevertheless, GZ do not always represent ‘trophic dead ends.’ Some predators directly consume GZ, and many species host parasitoids which in turn are prey for other organisms. We characterized trophic relationships between the fried-egg jellyfish Phacellophora camtschatica, its hyperiid amphipod parasite (Hyperia medusarum), and juvenile Chinook Salmon Oncorhynchus tshawytscha in the Salish Sea. Hyperia medusarum occurred in 29–47% of Chinook Salmon stomachs over 3 years and were observed in high abundance on P. camtschatica medusae (mean = 428 individuals per medusa). Light and transmission electron microscopy confirmed the presence of morphologically similar nematocysts in the foreguts of H. medusarum from both medusae and Chinook Salmon stomachs. Occurrence of H. medusarum in Chinook Salmon diets was also positively related to an index of P. camtschatica abundance. Chinook Salmon stomachs contained almost exclusively mature female H. medusarum while males and juveniles were common on medusae. Size and sex ratio differences between hyperiids in Chinook Salmon diets and on medusae could reflect predator selectivity or sex and/or life-stage specific differences in H. medusarum behavior. Our results support previous speculation that GZ abundance may facilitate predation on hyperiids by Pacific Salmon. Hyperiid-mediated energy flow from GZ to fish is not limited to medusa-associated fish species and should be considered in studies of marine food webs.